MECHANISM OF UPTAKE OF TECHNETIUM-TETROFOSMIN .1. UPTAKE INTO ISOLATED ADULT-RAT VENTRICULAR MYOCYTES AND SUBCELLULAR-LOCALIZATION

Background. Tc-99m-labeled tetrofosmin is a new myocardial imaging age nt that gives stable heart uptake. However, little is known about the mechanism of uptake in heart tissue. Methods and Results. Uptake of Tc -99m-labeled tetrofosmin has been examined in isolated adult rat ventr icular myocytes. The time course of uptake, efflux rate, and the effec t of metabolic and cation channel inhibitors have been assessed. The s ubcellular localization of radioactivity in ex vivo rat heart tissue w as examined by differential centrifugation of ventricular homogenate. Uptake into rat myocytes was found to be rapid and plateaued at simila r or equal to 1.5 pmol/10(6) cells/nmol extracellular Tc-labeled tetro fosmin after 60 minutes of incubation. Uptake was temperature dependen t but independent of extracellular Tc-labeled tetrofosmin concentratio n. Uptake at 30 minutes was inhibited by the metabolic inhibitors iodo acetic acid acid and 2.4-dinitrophenol protein but was not affected by cation channel inhibitors. Cells previously incubated with Tc-99m-lab eled tetrofosmin and then placed into fresh medium were found to have a slow efflux of activity; after 1 hour, 65% of activity was still cel l associated. The localization of radioactivity in subcellular fractio ns indicated that the majority of activity was recovered with the cyto sol. However, examination of the distribution of two mitochondrial enz ymes indicated that this may have been artifactual. Use of carbonyl cy anide m-chlorophenyl-hydrazone or oligomycin to perturb mitochondrial membrane potential decreased or increased recovery in the mitochondria l fraction, respectively. Conclusions. Tc-99m-labeled tetrofosmin upta ke by myocytes is by a metabolism-dependent process that does not invo lve cation channel transport. The most likely mechanism for this is by potential driven diffusion of the lipophilic cation across the sarcol emmal and mitochondrial membranes.